After a nice dip in the pool or relaxing soak in water, the pads on our hands and feet become quite pruney.

Why do our extremities react to water by wrinkling in such a way?

One group of researchers proposes a new theory, suggesting that pruney fingers may be the remnants of an adaptation in human ancestors that allowed them to grasp wet surfaces.

Acting similar to rain treads on tires, lead researcher Mark Changizi and colleagues hypothesize the groove-like indentations on the tips of fingers and toes channel out water when pressed to a surface, granting greater contact between the appendage and surface being touched.

Intriguingly, the pruney hand phenomenon seems to have neural roots. Previous research demonstrates that individuals with severed nerves in their fingers did not experience wrinkling after water exposure.

This finding suggests the nervous system - not the surrounding environment - produces the wrinkling response. Though more research is needed to confirm water wrinkling in other primate species, it's known that macaque monkeys get pruney digits as well.

In years past, people informally assumed that fingers and toes absorbed surrounding water in a type of osmotic exchange. Today, some scientists think dead keratin cells contribute to water uptake in the outermost layer of skin. They also consider the possibility of blood vessel constriction in the fingers, implying that nerve fibers in our digits and temperature-regulating passages through tissue shrink.

But there are setbacks to the research - one being the lack of data to support the greater utility of pruney fingers in comparison to non-wrinkled digits on wet surfaces. Simply put, it's not something that has been studied much. In a Nature News article, Changizi said testing the effectiveness of pruney hands in humans and nonhuman primates is the next step in exploring the theory, and preliminary results point to the wrinkles being advantageous.

One should also consider the fact that not all functions of the human body were "selected for" throughout our evolutionary history. What if pruney fingers came about indirectly from another trait and served no adaptive purpose?

For background, scientists' understanding of the archaeological and fossil records leads them to support that humans' ancestors probably evolved in a savanna-like environment with a mosaic of desert and patchy forest areas. It's likely encounters with water in Africa placed primate swimmers in unfavorable proximity to crocodiles as well.

The team makes no mention of the controversial aquatic ape theory - the idea that certain traits of our species result from a semi-aquatic past, but it definitely comes to mind.

Overall, pruney fingers in macaques and humans suggests two possibilities: The trait came to be in the last common ancestor of Old World monkeys and humans; or, it evolved independently in both species. It will be interesting to see where additional studies lead the hypothesis.

Until then, we'll have to bide our time pondering the "rain tread" hypothesis while examining our own wrinkly digits.

First photo by ohmeaghan/Flickr.com Second photo by PLoS Biology/Wikimedia Commons